1. Field of the Invention
This invention relates to a multi-port fiberoptic rotary joint for optically coupling through a trapezoid prism a plurality of optical fibers mounted on a rotative member and a fixed member of the rotary joint.
2. Background Art
The prior art will be described below with reference to the accompanying drawings wherein the like reference numerals designate identical elements in different Figures.
FIG. 1 illustrates a prior multi-port fiberoptic rotary joint disclosed in Japanese Utility Model Application Laid Open No. 61-6818 (6818/1986). As illustrated in FIG. 1, inside a fixed member 1 there are provided a rotative member 2, a trapezoid prism (dove prism) 3, and a prism holder 4 rotatable relative to and coaxial with the rotative member 2. Emission side optical fibers 5a and 5b are coupled with the entrance surface of the trapezoid prism 3 through ferrules 6a and 6b, and rod-shaped collimating convergent lenses 9a and 9b, both mounted on the fixed member 1. Reception side optical fibers 14a and 14b are coupled with the exit surface 3c of the trapezoid prism 3 through ferrules 13a and 13b, and collimating convergent lenses 10a and 10b, both mounted on the rotative member 2. Speed change gears 40, 41, 42, and a speed change gear shaft 43 are disposed among the rotator 2, the prism holder 4, and the fixed member 1, which serve in combination as a speed change gear mechanism for reducing the angular velocity of the rotative member 2 to half and for transmitting it to the prism holder 4.
Light from the emission side optical fiber 5a is collimated through the convergent lens 9a, allowed to enter the entrance surface 3a of the trapezoid prism 3, refracted there, and entirely reflected at the bottom surface 3b of the prism 3. And further, it is refracted at the exit surface 3c, permitted to go out thereof, collimated through the convergent lens 10b, and permitted to enter the receiving side optical fiber 14b. Another emission side optical fiber 5b and the receiving side optical fiber 14a are also coupled with each other in the same fashion discribed above. In the optical rotary joint, as the rotator 2 rotates, the trapezoid prism 3 rotates at an angular velocity which is half of that of the rotator 2 in the same direction, thereby optically cancelling the rotation of the rotator 2. Hence, couplings between the entrance and exit side optical fibers 5a and 14b, and between the entrance and exit side optical fibers 5b and 14a are not deteriorated.
FIG. 2 illustrates another prior example of a multi-port rotary joint as disclosed in Japanese Utility Model Application No. 60-157843 (157843/1985). In the multi-port fiberoptic rotary joint of FIG. 2, an improvement of the one illustrated in FIG. 1, there are provided intermediate optical fibers 8 and convergent lenses 9 between the emission side optical fibers 5 and a trapezoid prism 3, and likewise in the rotator 2 side there are provided intermediate optical fibers 11 and convergent lenses 10 between the receiving side optical fibers 14 and the trapezoid prism 3. Speed change gears 23, 24, 25 and 26, and a gear shaft 27 are provided among the rotator 2, the prism holder 4 and the fixed member 1 in the same manner as in FIG. 1 as the speed change gear mechanism for reducing the angular velocity to half of that of the rotator 2 and transmitting the decelerated rotation to the prism holder 4. The multi-port fiberoptic rotary joint is further adapted to couple the light from the emission side to the receiving side by means of a connector mechanism (a receptacle 18 and a plug 15) for connecting the emission optical fiber 5 with the optical fiber 8 and of another connector mechanism (a receptacle 19 and a plug 46) for connecting the receiving optical fiber 14 with the optical fiber 10.
The trapezoid prism 3 forms an inverted mirror image between an incident light image and an emergent light image with respect to an optical axis as illustrated in FIG. 1 when the length of the trapezoied prism 3, l, satisfies, assuming its aperture to be S, a relation l=4.23.times.S (BK-7, borosilicate glass, is employed as prism material). The outer diameters of the optical connectors (receptacles 15, 16, etc.) typically range from 6 to 10 mm, and hence in a four-core optical rotary joint for example, the aperture S.sub.1 =15 to 20 mm and the length l.sub.1 =63.5 to 84.6 mm.
However, as illustrated in FIG. 3, coupling loss between the optical fibers is remarkably increased as the distance between the convergent lenses goes beyond 50 mm. With l.sub.1 being between 63.5 mm and 84.6 mm, the coupling loss is increased to 3dB to 7dB. In addition, slight angular mismatching between the plugs 20a, 20b and the receptacles 15a, 15b causes a bundle of light to be expanded at the entrance surface 3a, bottom surface 3b, and exit surface 3c of the trapezoid prism 3 thereby deteriorating the rotation chracteristics of the multi-port rotary joint. In particular, the longer the length of the trapezoid prism 3, l, the more conspicuous the expansion of the bundle of light due to the angular mismatching. It is accordingly desirable to reduce the prism length l to the utmost.
To solve the aforementioned problem, a measure was taken as illustrated in FIG. 2, in which intermediate optical fibers 8 and 11 are interposed between the receptacles 18, 19 and the trapezoid prism 3 in order to improve the packaging density of the device. This however results in severe loss of the light when the intermediate optical fibers 8 and 11 are bent to an extreme, and requires a certain length because of the possibility of their being broken. For this, the total length L.sub.1 of the rotary joint must be large although the length l.sub.2 of the trapezoid prism 3 is reduced, making difficult their connection to a rotary part. Furthermore, the intermediate optical fibers 8 and 10 terminate at ferrules for connectors, whose ends therefore needs polishing, causing an increase in fabrication cost as well as an increase in the coupling loss because of the increase of connections by a factor of two.